1. Field of the Invention
This invention relates to a formation method for conductive bumps, and more particularly to a method for forming solder bumps or gold studs with two kinds of photo resist layers.
2. Description of the Prior Art
With the development of IC technology, the package of the IC is strictly required for the function of a product subjects to the technology of the package. The typical package, such as dual in-line package, QFP package, plastic flat package, and so on. The phenomenon of cross talk happens when the frequency of an IC is over 100 MHz. Furthermore, those traditional package technologies are not suitable for an IC with more than 208 pins. Now, most of the chips with the high pins are implemented by the technology of ball grid array package (BGA). Thus, the technology of ball grid array package becomes one of the best chooses for the packages of CPU, south/north bridge chips, and so on.
On the other hand, the technology of BGA includes multitudes of types, such as plastic BGA substrate, ceramic BGA substrate, flip chip BGA substrate, tape BGA substrate, and cavity down PBGA substrate. A typical package of IC includes single IC through the processes of leadframe or substrate, die attach, wiring, molding, trimming and forming, and so on. The packaged IC has a multiple size larger than the chip. Among them, for flip chip BGA, there are gold studs or solder bumps on an IC for soldering of PWB.
For example, FIGS. 1A through 1C are schematically cross-viewed diagrams illustrating the formation of solder bumps in accordance with a prior art. Shown on FIG. 1A, An under bump metallurgy layer 112(UBM layer) is on a wafer 110, which consists of an adhesive/diffusion barrier and a wetting layer for improved adhesion between solder balls and the surface. Next, shown in FIG. 1B, a photo resist layer 114 is formed on the under bump metallurgy layer 112. FIG. 1C shows that a pattern is transferred onto the photo resist layer 114 and then the photo resist layer 114 is patterned by lithography and etching.
However, used as the photo resist layer 114, a dry film is with a poor resolution as its thickness is on the increase. Thus, the thicker dry film is less capable of forming the solder bumps with the smaller pitches. Alternatively, a liquid photo resist layer used as photo resist layer 114 has a height subject to its fluidity. Thus, the thinner liquid photo resist layer is unsuitable to form the solder bumps with the thicker height. Furthermore, if a pattern with a high density is transferred onto the liquid photo resist layer, smaller openings would result in bridging.